the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Identification and ranking of volcanic tsunami hazard sources in Southeast Asia
- 1German Research Centre for Geosciences GFZ, Telegrafenberg, 14473 Potsdam, Germany
- 2German Aerospace Center DLR, Münchenerstr. 20, 82234 Wessling, Germany
- 3Geophysics Study Program, Department of Physics, Faculty of Mathematics and Natural Sciences, Universitas Gadjah Mada, Sekip Utara, Bulaksumur, Yogyakarta, Indonesia
- 1German Research Centre for Geosciences GFZ, Telegrafenberg, 14473 Potsdam, Germany
- 2German Aerospace Center DLR, Münchenerstr. 20, 82234 Wessling, Germany
- 3Geophysics Study Program, Department of Physics, Faculty of Mathematics and Natural Sciences, Universitas Gadjah Mada, Sekip Utara, Bulaksumur, Yogyakarta, Indonesia
Abstract. Tsunamis caused by large volcanic eruptions and flanks collapsing into the sea are major hazards for nearby coastal regions. They often occur with little precursory activity, and are thus challenging to detect in a timely manner. This makes the pre-emptive identification of volcanoes prone to causing tsunamis particularly important, as it allows for better hazard assessment and denser monitoring in these areas. Here, we present a catalogue of potentially tsunamigenic volcanoes in Southeast Asia and rank these volcanoes by their tsunami hazard. The ranking is based on a Multicriteria Decision Analysis (MCDA) composed of five individually weighted factors impacting flank stability and tsunami hazard. The data is sourced from geological databases, remote sensing data, historical volcano induced tsunami records and our topographic analyses, mainly considering the eruptive and tsunami history, elevation relative to the distance from the sea, flank steepness, hydrothermal alteration as well as vegetation coverage. Out of 131 analysed volcanoes, we found 19 with particularly high tsunamigenic hazard potential in Indonesia (Anak Krakatau, Batu Tara, Iliwerung, Gamalama, Sangeang Api, Karangetang, Sirung, Wetar, Nila, Ruang, Serua) and Papua New Guinea (Kadovar, Ritter Island, Rabaul, Manam, Langila, Ulawun, Bam), but also in the Philippines (Didicas). While some of these volcanoes, such as Anak Krakatau, are well-known for their deadly tsunamis, many others on this list are lesser known and monitored. We further performed tsunami travel time modelling on these high-hazard volcanoes, which indicates that future events could affect large coastal areas in a short time. This highlights the importance of individual tsunami hazard assessment for these volcanoes, dedicated volcanological monitoring, and the need for increased preparedness on the potentially affected coasts.
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Edgar U. Zorn et al.
Status: final response (author comments only)
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RC1: 'Comment on egusphere-2022-130', Anonymous Referee #1, 08 Apr 2022
The manuscript entitled “Identification and ranking of volcanic tsunami hazard sources in Southeast Asia” by Zorn et al. proposed a catalogue of potentially tsunamigenic volcanos in Southeast Asia and ranked these volcanoes by their tsunami hazards. The evaluation is based on a Multicriteria Decision Analysis (MDA) composed of five weighted factors. They identified 19 volcanoes with high tsunami hazard and 48 with moderate tsunami hazard. The proposed ranking system can identify the hazards of Anak Krakatau and Kadovar before a tsunami occurs as a retroactive study.
I agree that this study is meaningful to disaster mitigation of volcanic tsunamis. However, the ranking system proposed in this study is not objective and in lack of quantitively evidence to support the assessment. Meanwhile, the linear combination of five individually weighted factors for MDA are questionable. Unfortunately, the present form is not suitable for publication in Natural Hazards and Earth System Sciences. Significant additional work is required to improve the methodology and contents. My suggestion is that the manuscript should be revised substantially and resubmitted. I am willing to review this manuscript again after their revision. Here are my comments on this manuscript.
1. My first concern on this manuscript is that the ranking system is not objective. The scoring (F) is based on qualitive analysis. There is not physical or experimental evidence to prove the reasonability of such scoring. For example, the scoring of H/D-Ratio has values ranged from 0.02 to 0.89 and these values are multiplied by 100 to get a 0–100-point scale linearly. In that case, it means that a H/D-Ratio of 0.4 has twice the score (i.e., risk) to a value of 0.2. However, such assumption lacks evidence. No numerical simulation or geological evidence are presented to support the scoring method. This problem also occurs in other four factors.
2. Similarly, the weighting (W) of the ranking system is also subjective. I agree that the results of robustness testing are satisfactory. But the testing itself cannot show the importance (or contribution) of each factor for MDA. Therefore, the total weighted score can only be used as a rough estimation rather than a strict criterion. The authors may add a confidence level to each total weighted score.
3. The MDA of the ranking system is based on a linear combination of five individually weighted factors (Equation 1). However, these factors are not mutually independent. For example, a higher slope angle may result in a higher tsunami activity, and therefore, also increases the score of tsunamigenic history. The scoring and weighting of five factors may overlap, which is not appropriate to be represented by a linear combination.
4. The heat map (Figure 7) and travel-distance plots (Figure 8) cannot accurately represent the potential volcanic tsunami hazards because they do not incorporate the information of tsunami amplitude. It makes the hazard assessment less powerful. A tsunami with 1 m amplitude has evidently different impact from the one with 0.1 m amplitude. I believe it is a MUST to consider the potential maximum amplitude when analyzing volcanic tsunami hazards.
5. The conclusion of this manuscript is too simple. It is necessary to discuss the limitation of this ranking system.
Other minor comments:
Line 71: I agree that “the inherent problem of volcanogenic tsunamis is the lack of warning time and quick response options”. However, even if we successfully identified the high-tsunami-risk volcanoes, this problem still exists. Please discuss potential solutions (e.g., radar, bottom pressure gauges) to fix this inherent problem.
Line 105: Add a figure and use an example to show the process of defining the edifice boundary.
Line 332: Remove the repeated word in “high high-hazard”.
Line 333: Please explain the reason why there are some volcanoes with high scores but not prominently considered for their tsunamigenic potential.
Figure 5: Please add a subpanel to show the respective distribution between the countries for all considered volcanoes.
Figure 6: What are the different meanings between dark red and light red (also blue, yellow, green, etc.)? Please specify.
Section 4.3: This section seems verbose. The authors may present Batu Tara here and move others to supplementary material. Instead, it is better to have more discussions on potential tsunami scenarios of Batu Tara.
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RC2: 'Comment on egusphere-2022-130', Anonymous Referee #2, 30 Apr 2022
The manuscript submitted by Dr. Zorn and the colleagues shows the potential of tsunami hazards with volcanic origins in Southeast Asia (Indonesia, Papua New Guinea, Philippines, India, etc.). The authors focused on various factors of 131 volcanoes, such as topographic features, recent volcanic activity, tsunamigenic history in the past, which are considered closely related to tsunami potential, and used a Multicriteria Decision Analysis (MCDA) for the hazard assessment. Then, they found 19 with particularly high tsunami hazard potential, some of which less known and monitored.
While their assessment could not avoid their subjectivity in their definitions of the weights and the points in MCDA, the presented assessment that widely covers major volcanoes in this region is useful to consider tsunami potentials and for further consideration of volcanic tsunami potential at each volcano. I think this manuscript still has some parts to be improved, as listed below, but I believe that this manuscript has the potential to become suitable for publication from NHESS after major revisions.
[Major comments]
1. The objectivity of each factor used in MCDA
In MCDA, the authors considered different factors (H/D-Ratio, Volcanic activity, Tsunamigenic history, Slope angle, and Hazardous Features [Underwater extent, Morphological features, Vegetation, Hydrothermal alteration, Topography between an edifice and the sea]). I suppose that these factors are different in terms of objectivity and uncertainty; in other words, some are objective, while others contain error or subjectivity. For example, H/D-Ratio, slope angle, volcanic activity (if limited to recent activity), and underwater extent are based on rather reliable data. On the other hand, tsunamigenic history should contain many missing events (as the authors mentioned), morphological features cannot be simply quantitatively related to the hazard assessment, the effects of vegetations on edifice stability would depend on their type, etc... I recommend that the authors first use only “more objective” factors, and then add “less objective factors” (at least, please show results only with “more objective” factors, in the supplementary). It would be very helpful for readers’ understanding of what are the main factors determining the potential of volcanic hazards.
2. Similar factors in MCDA
Factors of morphological features and hydrothermal alternation seem to be related to the factor of volcanic activity. It seems that these related factors increase the scores for volcanoes that recently erupted. Please show how these factors are correlated with each other. If the correlations are large, some of the factors might be removed.
3. Potential spatial impact of volcanogenic tsunamis
The map in Fig. 7 does not add any important information, since the heat map of the volcanic tsunamis’ spatial impact shows high density around the high-hazard volcanoes, which is obvious. Also, the assessment of the spatial impacts only based on the tsunami travel times is disappointing. To consider the hazard, tsunami amplitudes on coasts should be taken into account. I understand that it is difficult to assume complex volcanic tsunami sources, the authors are recommended to conduct numerical simulations using linear long-wave models, at least with a simple tsunami source model (for example, a Gaussian-shape uplift on the sea surface).
[Minor comments]
Title: As the authors mentioned, submarine volcanoes are not considered in this study. Hence, it would be better to add such as subaerial volcanoes, volcanoes on land, or equivalent words to the title.
L35: causing some 26% of all volcano induced fatalities
This part is unclear. Do you mean “26% of all volcanoes causing tsunamis”? or 26 % of all volcanoes in the world (irrespective to tsunami generation)?
L88: although in some circumstances
In what circumstances do volcanoes inland exceed such a distance? Please mention some examples.
Figure 2:
Here the authors show only a case of Nila volcano in the high hazard category. At least, please show volcanoes in the other two categories for comparison.
L606– Conclusions
I recommend that the authors add one or a few sentences stating the limitations of this analysis (containing subjectivity and/or errors more or less).
Edgar U. Zorn et al.
Edgar U. Zorn et al.
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